1. Field of the Invention
The present invention relates to a communications system and a communication method. More particularly, the present invention relates to a communications system which enables communication between a central office and subscriber premises, and a communication method which enables communication between a central office and subscriber premises.
2. Description of the Related Art
A variety of telecommunication services are provided today, along with the growing needs for such systems as video on demand, cable TV, and high-speed access to computer networks. The development of subscriber access communications systems is thus underway to directly interconnect subscriber premises and central offices having databases for such services.
FIG. 17 is a diagram showing the structure of a subscriber access communications system. Termination unit 70-1 to 70-n are each disposed in subscribers' premises 700-1 to 700-n, respectively, while a line terminal 60 is disposed in a central office 600.
Each termination unit 70-1 to 70-n is linked to telephone sets, TV, personal computers, and other equipment. The line terminal 60 is linked to those termination units 70-1 to 70-n via a splitter 30 and a transmission medium.
Downstream information (downstream frames), directed from the central office 600 to the subscriber premises 700-1 to 700-n, is transported first through a single transmission line and then through a plurality of branching transmission lines via a splitter 30.
Upstream information (upstream frames) originating from the subscriber premises 700-1 to 700-n to the central office 600 is first transported over the individual branches and then combined into a single transmission lines via the splitter 30.
When broadcasting information in the above-described conventional subscriber access communications system, the line terminal 60 produces a downstream frame for this purpose by duplicating the data as many times as the number of termination units for which it is destined. Each copy of the data is then inserted to a particular time slot that is pre-assigned to every termination unit. The line terminal 60 sends out the downstream frame produced in this way.
FIG. 18 is a diagram outlining the process of copying data. The downstream frame is designed to have as many time slots as the number of termination units. The individual termination units 70-1 to 70-n are associated with those time slots TS1 to TSn, respectively. When sending data, the line terminal 60 produces n copies of that data and enters them into the time slots of a downstream frame.
The trouble is that the above process of creating and sending downstream frames for broadcasting is inefficient because the conventional system requires that the data be copied as many times as the number of termination units belonging to a group.
Another problem of the conventional system lies in the lack of flexibility in group communication control in the case the termination units 70-1 to 70-n are divided into a plurality of groups.